The present invention relates to rubber compositions for tires, and also to the antidegradants intended to protect these compositions against atmospheric photo-oxidizing aging due to the combined action of oxygen and light.
The invention relates more particularly to the anti-photo-oxidizing protection of white, clear or colored diene rubber compositions, which are devoid of carbon black, that are reinforced by at least one white or colored filler, in particular silica. Such compositions are sulphur-vulcanizable and form part of colored tires.
It is known that, vulcanized rubber compositions of essentially unsaturated natural and synthetic diene rubbers, because of the presence of double bonds in their molecular chains, are likely to deteriorate more or less rapidly after prolonged exposure to the atmosphere, if they are not protected, as a result of known oxidation mechanisms. These complex mechanisms have been described, for example, in the following documents: ref. [1]: xe2x80x9cAntidegradants for tire applicationsxe2x80x9d in xe2x80x9cTire compoundingxe2x80x9d, Education Symposium No. 37 (ACS), Cleveland, Communication I, October 1995; ref. [2]: xe2x80x9cNon-blooming high performance antidegradantsxe2x80x9d, Kautschuk Gummi Kunststoffe, Year 47, No. 4, 1994, 248-255; ref. [3]: xe2x80x9cAntioxidantsxe2x80x9d in Encycl. Polym. Sci. and Eng., 2d Edition, Vol. 2, 73-91. Following breaking of these double bonds and the oxidation of the sulphur bridges, oxidants bring about stiffening and embrittlement of the vulcanized rubber compositions, which degradation is furthermore accelerated under the combined action of heat by xe2x80x9cthermo-oxidationxe2x80x9d, or alternatively that of light by xe2x80x9cphoto-oxidationxe2x80x9d (see e.g. ref. [4]: xe2x80x9cPhotooxidation and stabilization of polymersxe2x80x9d, Trends in Polym. Sci., Vol. 4, No. 3, 1996, 92-98; ref. [5]: xe2x80x9cDegradation mechanisms of rubbersxe2x80x9d, Int. Polym. Science and Technol., Vol. 22, No. 12, 1995, 47-57).
It has been gradually possible to inhibit these oxidation phenomena as a result of the development and sale of various antioxidants, the most effective of which are, known derivatives of quinoline (xe2x80x9cTMQxe2x80x9d), or derivatives of p-phenylenediamine (xe2x80x9cPPDxe2x80x9d or xe2x80x9cPPDAxe2x80x9d), which are even more active than the former, such as, for example, N-1,3-dimethylbutyl-Nxe2x80x2-phenyl-p-phenylenediamine (6-PPD). These TMQ and PPD-type antidegradants, singly or in combination with each other, have found widespread systematically use (see, e.g., refs. [1] to [3] above) in conventional tire rubber compositions, filled at least in part with carbon black, which imparts thereto their characteristic black color.
Since savings in terms of fuel and the necessity of protecting the environment have become a priority, and in particular since the publication of European Patent Application EP-A-0 501 227, which corresponds to U.S. Pat. No. 5,227,425, the interest in silica-reinforced rubber compositions has been widely revived. The EP ""227 application describes a sulphur-vulcanizable rubber composition reinforced, preferably, by a special precipitated silica, which makes it possible to manufacture a tire having a substantially improved rolling resistance, without adversely affecting the other properties, in particular those of adhesion, endurance and wear resistance.
Recently published European Patent Application EP-A-0 810 258, which corresponds to U.S. Pat. No. 5,900,449, discloses a novel diene rubber composition reinforced by another special white filler, in this case a specific alumina (Al2O3) of high dispersibility, which also makes it possible to obtain tires or treads having such an excellent balance of various properties, as provided above.
Thus, it is henceforth possible to conceive of colored tires being sold which, for aesthetic reasons, in particular in the field of passenger vehicles, meet a real expectation of the users, while being able to provide them with a substantial saving in terms of fuel.
However, the antioxidants described previously, which have been developed and optimized after many years of research into conventional black rubber compositions, are not suitable for protecting tire rubber compositions filled exclusively with white fillers, in particular silica or alumina, because such antioxidants, and in particular the TMQ or PPD derivatives referred to above, are not light-fast. Under the action of UV radiation, they undergo an adverse color change and stain the rubber compositions, which would not allow their use in white, clear or colored compositions. Furthermore, the absence of carbon black, which hitherto acted as a very effective UV absorber in conventional compositions, results in aggravating all the degradation processes described previously, in particular those of photo-oxidation.
It was therefore necessary, in order to be able to develop colored tires, to develop new systems for protecting against photo-oxidizing aging, which improved properties over known antioxidants used for black tires, in particular being effective against UV, while being light-fast and non-staining with respect to the compositions to be protected.
The present invention provides an anti-photo-oxidizing protection system which meets the above demands, this system being compatible first with sulphur vulcanization and second with the harsh conditions of use of tires (temperature, fatigue due to dynamic stresses).
Consequently, a first aspect of the present invention is a white, clear or colored tire rubber composition devoid of carbon black, the composition comprising at least one diene elastomer, a white or colored reinforcing filler, and an anti-photo-oxidizing protection system, this composition being characterized in that said protection system comprises the following compounds A-D:
(A) a 2,2xe2x80x2-methylene-bis-[4-(C1 to C10)alkyl-6-(C1 to C12)alkylphenol];
(B) a dialkyl thiodipropionate, the alkyl radicals of which, which may be identical or different, are C1 to C30 radicals, preferably C8 to C20 radicals;
(C) a 2-(2-hydroxyphenyl)benzotriazole;
(D) a Hindered Amine Light Stablizer (xe2x80x9cHALSxe2x80x9d) amine derived from 2,2,6,6-tetramethyl piperidine.
A further aspect of the invention is directed to colored tires or colored rubber articles for such tires, comprising a rubber composition according to the invention. In particular, such articles include particular treads, underlayers intended, for example, to be placed beneath these treads, sidewalls, protectors, beads, and more generally any rubber layer or ply which may form part of a tire.
xe2x80x9cColoredxe2x80x9d tires or rubber articles are understood in the present description to mean tires or rubber articles, at least part of which is of a color other than the conventional black, including white.
A still further aspect of the invention provides a process for protecting a white, clear or colored tire rubber composition, devoid of carbon black, against photo-oxidizing aging, this process being characterized in that an anti-photo-oxidizing system, such as described above, is incorporated by mixing with said composition before the vulcanization thereof.
Finally, the invention is also directed to anti-photo-oxidizing system, as defined above, for protecting these white, clear, or colored tire rubber compositions against photo-oxidation.